The nuclear-encoded PsbW protein subunit of photosystem II undergoes light-induced proteolysis

The repair of photoinhibitory damage to photosystem II involves the rapid degradation and turnover of the D1 reaction center subunit. Additional protein subunits which show a limited degradation at high light intensities are the complementary reaction center subunit, D2, and the two chlorophyll a binding proteins, CP 47 and CP 43. In this work, we provide the first evidence for light-induced degradation of a nuclear-encoded subunit of photosystem II, the recently discovered PsbW protein. This 6.1 kDa protein is predicted to have a single membrane span and was found to be closely associated with the photosystem II reaction center. The degradation of the PsbW protein was demonstrated by photoinhibitory experiments, both in vitro, using thylakoid membranes and photosystem II core particles, and in vivo using leaf discs. The PsbW protein showed almost the same rate and extent of degradation as the D1 protein, and its degradation was more pronounced compared to the D2 and CP 43 proteins. The degradation of the PsbW protein was shown to share many mechanistic similarities with the more well characterized D1 protein degradation, such as oxygen dependence, sensitivity to serine protease inhibitors, and high light triggering while the actual degradation could readily occur in total darkness. The degradation of the PsbW protein was impaired by protein phosphorylation, although this protein was not itself phosphorylated. This impairment was correlated to the phosphorylation of the D1 protein which has been shown to block its degradation during photoinhibitory conditions. It is concluded that the PsbW protein is not degraded as a direct consequence of primary photodamage but due to a general destabilization of the photosystem II complex under conditions were the D1 protein becomes degraded in the absence of a sufficient repair system. The results are discussed in terms of a requirement for coordination between degradation and protein synthesis/integration during the repair process of photodamaged photosystem II reaction centers.